vacuumdb: Don't assign negative values to a boolean.
vacuumdb: Don't assign negative values to a boolean.

Since a17923204736 (vacuumdb: enable parallel mode) -1 has been assigned
to a boolean. That can, justifiedly, trigger compiler warnings. There's
also no need for ternary logic, result was only ever set to 0 or -1. So
don't.

Discussion: 13064aaaa855bf3b3a6a13291f56be176cea166a@awork2.anarazel.de
Backpatch: 9.5

zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl